But he's not an actor. And even though he may be able to do all his own stunts, they wouldn't let him; if the main actor gets injured doing a stunt, the movie is held up for months while he recovers.
How about all those musicians who receive roles in movies, despite no prior experience? A person doesn't have to be an actor to pull off a critically acclaimed performance.
Many roles are assigned based on a performers experience or occupation. Jet Li, Bruce Lee, Steven Seagal and Jean Claude Van Damme, were all competitive martial artists who have all been cast in movies as martial artists. Aquaman swims at standards which far exceed any normal person - Michael Phelps swims at standards which far exceed any normal person.
The fighting and dangerous underwater work could be done by pro-stuntmen, the special FX will be done with CGI, and Phelps could easily do the rest, should he show an aptitude for acting.
Maybe I'm just looking for realism in a very fictional role.